Been living around the world for three years now. Definitely recommend the Palermo Soho neighborhood of Buenos Aires. Great nightlife and food. Good cafés to work from. Spent several months in Rio - my gf is from there. Love the city and have been back several times, but very important to remain ultra-aware of your surroundings. Santiago is nice but I've no urge to go back. Montevideo is quite boring but the people are wonderful. Still have yet to go to Medellin, Colombia, but it's practically the digital nomad capital of SA. Heard great things about there and Cartagena.
Definitely learn some Spanish but Colombia comes to mind. Medellín is an up and coming place that I'd recommend. Also check out the coast if you can, Cartagena is tourist friendly. As for learning, start with some basics on duolingo and then come over to east San José and hang out in a Mexican bar, restaurant, etc. Plenty of opportunites to learn around here. Que te vaya bien.